Opioid drugs typically target three types of endogenous opioid receptors (i.e., μ, δ, and κ receptors) in biological systems. Many opiates, such as morphine, are μ opioid agonists that are often used as analgesics for the treatment of severe pain due to their activation of μ opioid receptors in the brain and central nervous system (CNS). Opioid receptors are, however, not limited to the CNS, and may be found in other tissues throughout the body, i.e., peripheral to the CNS. A number of side effects of opioid drugs may be caused by activation of these peripheral receptors. For example, administration of μ opioid agonists often results in intestinal dysfunction due to the large number of receptors in the wall of the gut (Wittert, G., Hope, P. and Pyle, D., Biochemical and Biophysical Research Communications, 1996, 218, 877-881; Bagnol, D., Mansour, A., Akil, A. and Watson, S. J., Neuroscience, 1997, 81, 579-591). Specifically, opioids are generally known to cause nausea and vomiting, as well as inhibition of normal propulsive gastrointestinal function in animals and man (Reisine, T., and Pasternak, G., Goodman & Gilman's The Pharmacological Basis of Therapeutics, Ninth Edition, 1996, 521-555), resulting in side effects such as, for example, constipation.
It would be of benefit to inhibit the natural activity of endogenous opioids during and/or after periods of biological stress, such as surgery and childbirth, so that ileus and related forms of bowel dysfunction can be prevented and/or treated. Currently, therapies for ileus include functional stimulation of the intestinal tract, stool softeners, laxatives, lubricants, intravenous hydration, and nasogastric decompression. These prior art methods suffer from drawbacks, for example, as lacking specificity for post-surgical or post-partum ileus. And these prior art methods offer no means for prevention. If ileus could be prevented, hospital stays, recovery times, and medical costs would be significantly decreased, in addition to the benefit of minimizing patient discomfort. Thus, drugs that selectively act on opioid receptors in the gut would be ideal candidates for preventing and/or treating post-surgical and post-partum ileus. Of those, drugs that do not interfere with the effects of opioid analgesics in the CNS would be of special benefit in that they could be administered simultaneously for pain management with limited side effects.
Peripheral opioid antagonists that do not cross the blood-brain barrier into the CNS are known in the literature and have been tested in relation to their activity on the GI tract. In U.S. Pat. No. 5,250,542, U.S. Pat. No. 5,434,171, U.S. Pat. No. 5,159,081, and U.S. Pat. No. 5,270,328, peripherally selective piperidine-N-alkylcarboxylate opioid antagonists are described as being useful in the treatment of idiopathic constipation, irritable bowel syndrome, and opioid-induced constipation. Some peripheral μ antagonists derived from the structure naltrexone have been reported in the literature (U.S. Pat. No. 4,806,556; Botros, et al., J. Med. Chem. 1989, 32, 2068-2071). In addition, U.S. Pat. No. 4,176,186 describes quaternary derivatives of noroxymorphone (i.e., methylnaltrexone) that are said to prevent or relieve the intestinal immobility side effect of narcotic analgesics without reducing analgesic effectiveness. U.S. Pat. No. 5,972,954 describes the use of methylnaltrexone, enteric-coated methylnaltrexone, or other quaternary derivatives of noroxymorphone for preventing and/or treating opioid- and/or nonopioid-induced side effects associated with opioid administration.
General opioid antagonists, such as naloxone and naltrexone, have also been implicated as being useful in the treatment of GI tract dysmotility. For example, U.S. Pat. No. 4,987,126 and Kreek, M. J. Schaefer, R. A., Hahn, E. F., Fishman, J. Lancet, 1983, 1, 8319, 261 disclose naloxone and other morphinan-based opioid antagonists (i.e., naloxone, naltrexone) for the treatment of idiopathic gastrointestinal dysmotility. In addition, naloxone has been shown to effectively treat non-opioid induced bowel obstruction, implying that the drug may act directly on the GI tract or in the brain (Schang, J. C., Devroede, G., Am. J. Gastroenerol., 1985, 80, 6, 407). Furthermore, it has been implicated that naloxone may provide therapy for paralytic ileus (Mack, D. J. Fulton, J. D., Br. J. Surg., 1989, 76, 10, 1101). However, it is well known that activity of naloxone and related drugs is not limited to peripheral systems and may interfere with the analgesic effects of opioid narcotics.
Despite recent advances in peripherally acting opioids, there is still a need for more effective and safe opioid compounds, with potent μ, κ, and/or δ receptor agonist activities that produce essentially no central mechanism side effect, more particularly for the use in preventing or treating undesirable side effects associated with administering exogenous opioids with minimal impact on opioid agonist analgesia.